devil in disguise

1.
generalidiomaticinformal
- A person who appears harmless, virtuous, or attractive but in reality is malicious, deceitful, or morally corrupt.
- Someone whose outward appearance conceals harmful intentions or character.
2.
generalcolloquialplayful
- A light or teasing way to describe a person who is mischievous or naughty while appearing innocent.
- A figurative label applied to things or situations that seem benign but turn out to be dangerous or troublesome.

Examples: